The present invention relates to a mobile radio communication system employing a time division multiple transmission scheme.
Conventionally, as described in a publication entitled "PERSONAL HANDY PHONE SYSTEM, RCR STANDARD (PROPOSAL), VERSION 1" edited by Research & Development Center for Radio System and issued Dec. 20, 1992, particular slots in a frame are only used to transmit a common control signal.
If a variety of radio communication systems are simultaneously operated in the same communication area in the same frequency band, the systems possibly interfere with each other in communications. For this reason, in order to permit a variety of radio communication systems to operate in the same communication area in the same frequency band, it is necessary to perform carrier sense, before a station belonging to one of the systems transmits a radio wave, for confirming whether or not the frequency of the radio wave and time-divided channels at that frequency are currently being utilized by any other system for communications in the area to confirm that an empty channel is still available at that frequency and start communications on that empty channel.
The carrier sense lasting about 10 milliseconds (ms) performed before a station emits a radio wave is described in FCC, Part 15 Rules DA93-1278, Oct. 22, 1993, .sctn.15. 323 (c)-(1).
However, even if a radio communication system starts communications on the confirmed empty channel at that frequency, this radio communication system could be interfered by an invasion of communication signals generated by another radio communication system which would start communications in the same frequency band after the former radio communication system has started the communications. Even if the former radio communication system continues the communications while continuously performing the carrier sense after it has started the communications in order to avoid such interference, since the carrier sense is performed only for a short time of approximately 10 ms, a radio communication system which transmits information with a frame of a longer time than the carrier sense period, i.e., 10 ms, may not be detected within the carrier sense period. Consequently, this allows the other radio communication system to invade the same frequency band, whereby the interference similar to the above cannot be prevented.
Also, in a radio communications system which provides a plurality of cells, each of which defines a communication area for a base station, a common control signal for commonly controlling the plurality of cells is required to enable personal stations to easily and rapidly capture the common control signal, even if the personal stations move from a cell to another. For achieving this requirement, the common control signal need be transmitted among these cells on a channel at the same frequency but in different time slots.
However, when all time slots at a certain frequency, which is supposed to be assigned to the common control signal, have already been occupied by another communication system in a cell, the whole radio communication system is prohibited from utilizing the frequency. For avoiding such inconvenience, a plurality of carrier frequencies may be provided such that an available frequency is selected among them for use in communications. This approach, however, forces personal stations to search different frequencies for their common control signal, with the result that excessively long time is required to detect the common control signal.
U.S. patent application Ser. No. 08/350,664, now U.S. Pat. No. 5,586,122, which is a related to the present application, describes an approach for preventing invasion from other radio communication systems, as described above, wherein a pilot signal is transmitted for claiming a priority right, i.e., claiming that a channel assigned to a base station belonging to a system is currently in use so that other radio communication systems cannot invade it to ensure the channel, such that stable communications are maintained.
However, if respective base stations belonging to various radio communication systems transmit their respective pilot signals to attempt to occupy a channel, the respective base stations transmit their own pilot signals on different channels from each other. The same is also applied to even base stations belonging to adjacent cells in the same radio communication system. In such a situation, although the communications can be controlled on a single channel because these base stations are located in adjacent cells in the same system, a plurality of channels are occupied by them. As a result, not only personal stations are obliged to spend longer time for a search for capturing the common control signal, but also resources including limited frequencies and time slots assigned to the communications are spent for nothing. Apparently, this circumstance is not favorable in view of the whole communication system.